The present invention relates to a coating apparatus having a coating liquid supplying slit, for example, a slide bead coater, curtain coater, or extrusion coater, and more particularly to a coating apparatus in which occurrence of coating streak defects is greatly reduced.
In many cases, coating troubles, specifically streak defects occur along the length of a long roll of web material. Accordingly, it greatly influences the quality or the production yield. Recently, technology of a multi-layer simultaneous processing method has been developed for the manufacture of silver halide photographic sensitive material in which many thin layers are required, wherein multi-layer means a plurality of layers.
For these multi-layer simultaneous coating methods, a coating apparatus such as a slide hopper, curtain coater, or extrusion coater is preferable. However, a problem of streak defects tend to occur in the use of such coating apparatus.
Streak defects are caused by various factors. Primary factors of the streak defects are: the disturbance of beads; the invasion of foreign matter; or the deposit of foreign matter to a portion of the coating apparatus.
These defects possibly occur in a mono-layer. However, specifically in a coating solution layer such as color sensitive material, when a portion of a color layer is scratched for some reason, streak-like thin layer portions are formed. When different color layers are simultaneously coated on an upper layer of the above-described portions, a difference of concentration occurs, and visibly appears as streak defects, resulting in a considerable problem.
Conventionally, a coating bridge, formed between the end of a coater lip and a web, so-called disturbance of beads is mainly responsible for the above-described defects. In order to solve the above-described defects, many methods including the following are proposed: the pressure reduction method disclosed in U.S. Pat. No. 2,681,294 and its improvement; the improvement of the process at the time of passage on joints disclosed in Japanese Patent Publication No. 4371/1973; considerations of the type of a lip and a slide surface angle disclosed in Japanese Patent Publication No. 39980/1976; the improvement of the surface roughness of the lip surface disclosed in Japanese Patent Publication Open to Public Inspection No. 207865/1990; and other methods and improvements.
Further, regarding the shape of a slit, the technology for providing a cutout-step portion, and further for providing an enlarged portion or cutout in the slit is disclosed in U.S. Pat. No. 3,005,440 or Japanese Patent Publication Open to Public Inspection 328174/1992 filed by the present by inventors.
However, the causes of streak defects are not limited to the above-described problems, and these defects have not been yet solved.